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FreeBSD-SA-17:08.ptrace                                     Security Advisory
                                                          The FreeBSD Project

Topic:          Kernel data leak via ptrace(PT_LWPINFO)

Category:       core
Module:         ptrace
Announced:      2017-11-15
Credits:        Ilja van Sprundel
                John Baldwin
Affects:        All supported versions of FreeBSD.
Corrected:      2017-11-10 12:28:43 UTC (stable/11, 11.1-STABLE)
                2017-11-15 22:39:41 UTC (releng/11.1, 11.1-RELEASE-p4)
                2017-11-15 22:40:15 UTC (releng/11.0, 11.0-RELEASE-p15)
                2017-11-10 12:31:58 UTC (stable/10, 10.4-STABLE)
                2017-11-15 22:40:32 UTC (releng/10.4, 10.4-RELEASE-p3)
                2017-11-15 22:40:46 UTC (releng/10.3, 10.3-RELEASE-p24)
CVE Name:       CVE-2017-1086

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

0.   Revision history

v1.0   2017-11-15  Initial release.
v1.1   2017-11-20  Corrected credit. Ilja van Sprundel first reported this
                   issue to the project, but wasn't cited. The FreeBSD
                   Security Team apologizes to Ilja for this oversight.

I.   Background

The ptrace(2) syscall provides the facility for a debugger to control the
execution of the target process and to obtain necessary status information
about it.  The struct ptrace_lwpinfo structure is reported by one of the
ptrace(2) subcommand and contains a lot of the information about the stopped
thread (light-weight process or LWP, thus the name).

II.  Problem Description

Not all information in the struct ptrace_lwpinfo is relevant for the state
of any thread, and the kernel does not fill the irrelevant bytes or short
strings.  Since the structure filled by the kernel is allocated on the
kernel stack and copied to userspace, a leak of information of the kernel
stack of the thread is possible from the debugger.

III. Impact

Some bytes from the kernel stack of the thread using ptrace(PT_LWPINFO)
call can be observed in userspace.

IV.  Workaround

No workaround is available.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

Afterward, reboot the system.

2) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Afterward, reboot the system.

3) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-17:08/ptrace.patch
# fetch https://security.FreeBSD.org/patches/SA-17:08/ptrace.patch.asc
# gpg --verify ptrace.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r325643
releng/10.3/                                                      r325871
releng/10.4/                                                      r325870
stable/11/                                                        r325642
releng/11.0/                                                      r325869
releng/11.1/                                                      r325868
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-1086>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-17:08.ptrace.asc>
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